The present application relates to a display device and a method of driving the same and, more particularly, to a technique for driving a defective element (measures for repairing the same in practice).
Various types of electronic apparatus include display devices which utilize electro-optical elements having brightness changing with a voltage applied thereto or a current passed therethrough as pixel displaying elements. For example, liquid crystal display elements are typical electro-optical elements having brightness changing with a voltage applied thereto. Typical examples of electro-optical elements having brightness changing with a current passed therethrough include common LEDs (light emitting diodes) and organic electro luminescence elements and organic light emitting diodes (OLEDs) which will be hereinafter collectively referred to as organic EL elements. Display devices utilizing the latter elements, i.e., common LEDs or organic EL elements, are so-called self-emitting display devices which utilize self-emitting electro-optical elements as a display element of a pixel.
An electro-optical element is made to emit light as follows. For example, in the case of an active matrix type element, an input image signal supplied through a video signal line is stored in a holding capacity (which may alternatively be called a pixel capacity) provided at a gate end (control input terminal) of a driving transistor using, for example, a switching transistor (which may be called a sampling transistor), and a drive signal is supplied to the electro-optical element according to the stored input image signal. In the case of a passive matrix type element, an electro-optical element is disposed in an intersection between a column scan line and a row scan line, and the electro-optical element is driven by drive signals supplied to the column scan line and the row scan line.
In a liquid crystal display device utilizing liquid crystal display elements as electro-optical elements, since the liquid crystal display elements are voltage-driven elements, a voltage signal according to an input image signal stored in a holding capacity is used as it is to drive a liquid crystal display element. On the contrary, in an organic EL display device utilizing current-driven elements such as organic EL elements as electro-optical elements, a drive signal (voltage signal) according to an input image signal stored in a holding capacity is converted into a current signal using a drive transistor, and the drive current is supplied to an organic EL element or the like.
However, an electro-optical element may become an improperly emitting pixel for some cause associated with the manufacture of the panel or the way the panel is used after shipment. Thus, panels may have defective elements which can reduce the yield of the panels. Such display defects constitute a factor hindering the improvement of the non-defect ratio of display devices, and a cost reduction of the display devices is consequently hindered. Defects can also occur after the shipment of a product, and the display quality of the product is degraded.
A defective state of an electro-optical element appears in the form of an unlit dot (a dark dot or non-emitting pixel), a bright dot (a pixel emitting with high brightness out of a normal range), or a dot emitting at an insufficient level of emission out of the normal range depending on the type of the element (for example, depending on whether the element is a liquid crystal display element or an LED element which may be an organic EL element) or depending on the type of the defect, e.g., depending on whether the defect is a short circuit defect, an open circuit defect, or an insufficient state of driving.
Under the circumstance, some approaches are taken to cope with defects of a display device used in an electronic apparatus by inspecting each of pixels arranged on a display panel to find any defective state (see JP-A-2003-262842 (Patent Document 1) and JP-A-2005-274821 (Patent Document 2)).
For example, Patent Document 1 discloses the use of a method in which a defective element of a display panel is repaired by reworking the panel directly. Specifically, a bright dot defect of a liquid crystal display device is corrected using a repair method in which the defective element is irradiated with a laser beam to rework an alignment film thereof such that the liquid crystal aligning performance of the film will be moderated to reduce the quantity of transmitted light.
Patent Document 2 addresses light-emitting display panels in which a multiplicity of pixels each including a self-emitting element having diode characteristics are arranged in the form of a matrix at intersections between scan lines and data lines and in which each of the self-emitting elements is selectively driven for emission. According to the technique, any problem in each self-emitting element of a light emitting display panel is detected, and detection results are stored in storage means. Locations having a problem (defect) are thus identified, and defect notification means is activated accordingly. Thus, a user can be quickly notified of the presence of a defective element.
The approach disclosed in Patent Document 1 has a risk of a new defect attributable to the use of a laser beam, and concern exits also about a possible cost increase attributable to a great amount of man-hour required for the rework. Let us assume that an element, electrode, or wiring of a display panel having electrodes formed by a matrix of self-emitting electro-optical elements such as LEDs is broken using a laser beam according to the method. Although a bright dot defect can be surely turned into a dark dot, another problem arises because the element now has an open circuit failure. Specifically, when the element has an open circuit failure, a current instantaneously flows through other elements connected in the same row in an amount equivalent to a floating capacity that those elements have. Thus, those elements may emit light at unwanted timing depending on the value of the current. After the floating capacity is charged with the current, a voltage increase approaching the circuit voltage of a current output circuit occurs, and the risk of application of an overvoltage therefore arises.
According to Patent Document 2, when a pixel has a defect, a user is notified of the defective state to prevent wrong display information from being conveyed to the user. Although the document addresses anti-defect measures, no measure against a defective element itself is disclosed.